1. Field of the Invention
This invention relates to a television tuner to which television signals of a plurality of channels arranged in a predetermined frequency band are supplied, and more particularly relates to a television tuner that performs frequency conversion twice.
2. Description of the Related Art
The structure of a television tuner is shown in FIG. 5. Television signals of a plurality of channels arranged in a band, for example, from 50 MHz to 800 MHz are supplied to a band pass filter 41, and needless signals outside the band are removed. The pass band of the band filter 41 is the same band from 50 MHz to 800 MHz as the band in which the television channels are arranged. The television signal that pass the band pass filter 41 is amplified by means of a high frequency amplifier 42, and the television signal of a receiving channel is frequency-converted to the first intermediate frequency signal of approximately 1300 MHz by means of the first mixer 43. A local oscillation signal to be served for frequency conversion is supplied from a first local oscillator 44 to the first mixer 43. A first PLL circuit 45 controls the oscillation frequency of the first local oscillator 44.
The first intermediate frequency filter 46 having the pass band of approximately 6 MHz removes needless signals from the first intermediate frequency signal, and then a second mixer 47 frequency-converts it to the second intermediate frequency signal of approximately 45 MHz. A second local oscillator 48 supplies the local oscillation signal to be served for frequency conversion to the second mixer 47, and a PLL circuit 49 controls the oscillation frequency of the second local oscillator 48. The second intermediate frequency signal passes a second intermediate frequency filter 50 and is amplified by means of a second intermediate frequency amplifier 51, and then supplied to an intermediate frequency circuit not shown in the drawing.
In the case of a conventional television tuner, because television signals of all channels are supplied to the high frequency amplifier 42, the high frequency amplifier 42 generates many strained signals, and these strained signals interfere with the television signal of the receiving channel, which is a problem.
For example, when the television signal of another channel having a xc2xd frequency of the television signal of a receiving channel is supplied to the high frequency amplifier 42, the double higher harmonic wave of the television signal of another channel is generated to with the television signal of the receiving channel because both frequencies are identical.
When the television signal of a different channel having a frequency to be doubled is supplied, the high frequency amplifier 42 generates a signal having a frequency that is the difference between the frequency of the television signal of the different channel and the frequency of the television-signal of the receiving channel, the frequency of the signal of the difference is identical with the frequency of the television signal of the receiving channel, and the signal interferes the television signal of the receiving channel. Needless signals such as higher harmonic waves and difference signals are generated also from the first mixer in the same, mechanism as involved in the high frequency amplifier 42.
It is the object of the present invention to prevent the needless signal that causes interfere as described hereinabove from being supplied, and to eliminate the generation of interfering signal.
To solve the above-mentioned problem, a television tuner of the present invention is characterized in that the television tuner is provided with a band switching circuit to which television signals of a plurality of channels arranged in a predetermined frequency band, the band switching circuit has a plurality of filters having the cutoff frequency of every one octave frequency in the predetermined frequency band, the cutoff frequency of the plurality of filters is made continuous every one octave in the predetermined frequency band, and the television signal of the channel to be received is sent out through the specified filter having the cutoff frequency within one octave from the frequency of the television signal of the channel to be received.
The television tuner is characterized in that the cutoff frequency can be changed by one octave.
The television tuner is further characterized in that the plurality of filters are provided with varactor diodes that constitute the filters, and a voltage is applied on the varactor diodes to vary the voltage.
The television tuner is further characterized in that input side switching diodes are connected respectively to respective input terminals of the plurality of filters and output side switching diodes are connected respectively to respective output terminals of the plurality of filters, a voltage is applied on the input side switching diode and the output side diode that are connected to the specified filter and the input side switching diode and the output side diode are made conductive.
The television tuner is characterized in that the television tuner is provided with a voltage dividing circuit having two voltage input terminals for generating a divided voltage that is different depending on the voltage applied on any one of the two voltage input terminals and provided with a band switching control circuit having a plurality of voltage output terminals for applying a switching voltage on at least one of the voltage output terminals corresponding to the channel to be received, the switching voltage applied on the voltage output terminals is applied on the input side switching diodes, the output side diodes connected to the plurality of filters, and the two voltage input terminals of the voltage dividing circuit, and the divided voltage is applied on the varactor diodes.
The television tuner is further characterized in that the plurality of filters are connected with high pass filters or low pass filters.
The television tuner is characterized in that the plurality of filters are constituted with band pass filters.